Alternating current ion-filter for electrical precipitators



June 29, 1954 WHITE v 2,682,313

ALTERNATING CURRENT ION-FILTER FOR ELECTRICAL PRECIPITATORS Filed Oct. 29, 1952 5 Sheets-Sheet l HARRY J. WHITE F157. 1 BY //M/ 2'M ATTORNEY H. J. WHITE ALTERNATING CURRENT ION-FILTER FOR ELECTRICAL PRECIPITATORS June 29, 1954 3 Sheets-Sheet 2 Filed Oct. 29, 1952 INVENTOR. HARRY J. WHIT E ATTORNEY June 29, 1954 Filed Oct. 29. 1952 J. WHITE 2,682,313

H. ALTERNATING CURRENT ION-FILTER FOR ELECTRICAL. PRECIPITATORS 3 Sheets-Sheet 3 Q o 9 l "X" 5f 6 n w L A L: Ml i I i, 2 .m

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HARRY J. WHITE BY A Wo TM ATTORNEY Patented June 29, 1954 UNITED STATES PATENT OFFICE ALTERNATING CURRENT ION-FILTER FOR ELECTRICAL PRECIPITATO'RS Harry J. White, Bound Brook, N. J., assignor to Research Corporation, New York, N. Y., a corporation of New York Application October 29, 1952, Serial No. 317,529

8 Claims. 1

This invention relates to a method and ap 'paratusforthe charging of solid and liquid particles'suspended in a gas.

Electrical precipitators are known in which a gas containing suspended particles is passed through a high potential electric field maintained between an attenuated or discharge electrode and an extended surface .or non-discharge electrode. Ionization .of the gas by the action of corona or silent discharge from the attenuated electrode brings about charging of the particles. Thecharged particles are transported by the action of the electric field in which the charging is effective, in the direction from the attenuated "or discharge electrode toward the extended sur-- face or collecting electrode upon or near which they are deposited. The particles fall from this electrode by gravity into a suitably placed hopper or :else :are periodically shaken or rapped, which causes them to fall into the collecting hopper. The corona discharge which ionizes the particles is produced in the vicinity of the attenuated electrodes due to the local high voltage gradient which exists in this region, the sharp curvature or the attenuated electrodes being a factor in the production of this high voltage gradient. The extended electrode has a smooth surface so that :there is no local corona discharge in the vicinity of this electrode. Under some condition, however, a corona discharge does occur at the collecting electrode. This .is termed back corona and is highly undesirable, as it greatly reduces the efliciency of the precipitating operation. The conditions under which back corona arises vary greatly with the character of the deposited material and the chemical composition and physical :state of the suspending gas, but whenever back corona is encountered a great reduction in the charging 'efiiciency results, due to the formation of :ions of opposite polarity to those formed in the discharge from the attenuated electrode, to

localization of the corona discharge, and to other factors.

One important factor in the production of this undesirable back corona is the presence of a relatively large number of gas ions which succeed in reaching the collecting or ground electrode. This results in relatively large ion currents which pass through the dust layer which normally accumulates on the ground electrode and thereby cause back corona trouble, particularly when treating high-resistivity dispersoids.

It is a primary object of this invention to provide a method and means whereby such gas ions are selectively removed from the gas stream before reaching the collecting or ground electrodes, thus obviating the above difiiculty. This is accomplished in a simple and effective manner, by providing a set of auxiliary electrodes in the region between the discharge and the collecting main electrodes, and preferably near the collecting electrode. A non-discharging alternating potential is established between adjacent ones of these auxiliary electrodes at a frequency which is too high to materially affect the relatively slow-moving dust particles, but sufficiently high to effectively remove or filter the gas ions due to the much greater mobility of the latter. A D. C. potential is also established between the auxiliary electrodes and the main electrodes and. is so adjusted that most of the D. 0. lines of force from the discharge wires and the ion-space charge pass through the ion filter to the collecting electrodes and is, in general, so adjusted that the D. C. potential of the auxiliary electrodes will be more negative than the normal corona discharge would be at the same point of distance from the discharge wires. This will also cause the D. C. field at the ground or collector electrodes to be enhanced, which is in itself a desirable eilect.

The specific nature of the invention, as well as other objects and advantages thereof, will clearly appear from a description of a preferred embodiment as shown in the accompanying drawing in which:

Fig. 1 is a schematic diagram showing the essential principles of the invention;

Fig. 2 is a diagrammatic side elevation view of .a precipitator arrangement embodying the inas is customary in this art. The collector electrode B which is grounded and usually positive in polarity is the wall of the duct. The nondischarging electrode system a, b is interposed between the discharge and collecting electrodes as shown. All the electrodes at, indicated by open circles, are of the same polarity and all of the electrodes h, indicated by black circles, are of the opposite polarity. The usual high voltage rectifier indicated at 20 is a component of the customary high voltage system for producing 0 unidirectional discharge voltage between main electrodes A and B. A separate alternating current source is provided by transformer 22 for setting up the ion-filter field between electrodes a and b. In addition, a unidirectional biasing potential is provided by rectifier 24.

The alternating current frequency of source 22, as previously stated, must be set too high to materially affect the slow moving dust particles, but not too high to effectively remove or filter the gas ions. This frequency range may be calculated from the known mobilities of the ions and the dust particles, in accordance with the following wellknown formula:

f=g l0 =3200 cycles per second The mobility of the dust particles may be taken in the order of 0.01. Applying the same formula, f=2 0.1 10 =32 cycles per second, which is the maximum value which will collect all of the dust particles. Both of these limiting values, of course, are approximate, and will vary somewhat with conditions. It is clear, however, that there will be available a wide range of frequencies intermediate the limiting frequencies which will be effective at least to some extent to produce the desired result. In any particular installation, an optimum frequency will exist which is preferably found by actual trial. It is clear, in any case, that the spacing and location of electrodes a, I), must be chosen so as to conform to the above requirements. The frequency must, of course, also be selected with the same consideration in mind.

Figs. 2 to 4 illustrate a practical embodiment of the invention. The precipitator is housed in a shell or housing I which is grounded and serves as part of the collector or extended surface electrode. An insulator housing 2 is provided above the precipitator shell I to house the insulating supporting elements for the precipitator system. A gas inlet duct shown at 3 is provided with distribution vanes 30. to distribute the incoming gases uniformly through the precipitator elements. The discharge elements A of the precipitator system are supported by a series of high tension busses 4 which rest on support beams l which are in turn supported by high tension insulators I0. Between the rows of discharge electrodes are placed extended surface collector electrodes B which are supported on suitable beams 'l'. The ion-filter electrodes a are supported by a high tension member 5 as best shown in Fig. 2; the ion-filter electrodes b are similarly supported by high tension members 6. The bottom ends of electrodes a and b are similarly supported by members l4 and i5,

respectively. Discharge electrodes A are provided with the customary weights I3 as best shown in Figs. 3 and 4.

It will be noted that the array of electrodes (1, b, nearest the walls of the housing I, extends only between the adjacent row of discharge electrodes A, and the outermost collector electrode B, since obviously there is no need for a set of filter electrodes between the outermost collector electrode B and the adjacent wall of the housing, since both are at ground potential.

Ion-filter electrodes a, b, may be in the form of small rods of approximately inch in diameter, placed approximately inch center to center. The peak alternating current voltage required in accordance with the above formula would then be about 10 kilovolts. The electrodes may suitably be located at about threefourths of the distance across the corona gap toward the ground electrodes. The electrical connections are substantially the same as shown in Fig. 1.

Due to the voltage established by rectifier 20 between electrodes A and B, there is a potential gradient in the space between the main electrodes. Thereafter, the filter system a, b, would, during normal operation, be at a certain D. C. potential above ground due to this gradient, as the filter system is located in this gradient.

It will be noted that the high-voltage rectifier 24 is so arranged as to establish a D. C. potential between all of the filter and ground. The value of this D. C. potential is set nearer to the value of the potential of discharge electrode A than would be provided at the locus of the filter by the normal potential gradient between electrodes A and B, is so adjusted that most of the D. C. lines of force from the discharge wires A and the ion-space charge pass through the filter a, b, to the collecting electrodes B. Since the potential difference between electrodes 0., b, and electrode B is thus greater than it would otherwise be, the D. C. field at the collector electrode is thereby enhanced, which tends to increase its effectiveness.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claims.

I claim:

1. The method of electrical precipitation which comprises passing a gas containing suspended particles through a unidirectional electric field produced between a discharge electrode and a collector electrode to produce gas ions and charged particles, both of which are attracted toward the collecting electrode, and selectively removing the gas ions before they reach the collecting electrode by means of an alternating field ion filter located between said electrodes.

2. The method of charging particles which comprises passing a gas containing suspended particles through a unidirectional electric field produced between an ionizing main discharge electrode and a non-discharge main collecting electrode to produce gas ions and charged particles both of which are attracted toward the collecting electrode, and selectively removing the gas ions before they can reach the collecting electrode by establishing an independent nondischarge alternating filter field in a localized region adjacent said collecting electrode at a frequency sufiiciently high with respect to the voltage gradient and to the boundaries of said ionfilter field to pass charged particles through said field to said collecting electrode, and sufiiciently low to trap gas ions from the discharge area.

3. The invention according to claim 2, said A. C. field being at a D. C. potential nearer to that of the discharge electrode than is provided by the normal D. 0. gradient between the main electrodes, whereby most of the D. C. line of force pass through the ion filter area to the collecting electrode.

4. Apparatus for charging particles which comprises means for passing gas containing suspended particles through a charging area; means for producing a charging field in said area comprising a discharge electrode system and an extended area collecting electrode system; means for establishing a high potential unidirectional voltage between said electrode systems to produce gas ions and charged particles both of which are attracted toward the collecting electrode system; and an ion filter for selectively removing the gas ions before they reach the collecting electrode system comprising a second set of spaced electrodes in the field between said main electrode systems and adjacent said collector electrode system, alternate electrodes of said second set being of opposite polarity; means for establishing an alternating electric field between the electrodes of opposite polarity of said second set, the voltage and frequency of said last field and the spacing of said second set of electrodes from each other being so related as to trap ions from the discharge area between said main electrodes prior to their reaching the collecting electrode, and to pass larger charged particles through said filter to said collecting electrodes.

5. An electrical precipitator comprising a charging system including two spaced insulated main electrodes, one being a discharge electrode and the other a non-discharge extended surface electrode; means for passing gas containing suspended particles between said electrodes; means for establishing a unipolar high-potential voltage between said electrodes; an ion filter comprising a second set of spaced electrodes in the field between said main electrodes, said second set comprising two groups of electrodes of opposite polarity; and means for establishing an alternating electric field between the electrodes of opposite polarity of said second set, the voltage and frequency of said last field and the spacing of said second set of electrodes from each other being so related as to trap ions from the discharge area between said main electrodes prior to their reaching the collecting electrode, and to pass larger charged particles through said filter to said collecting electrode.

6. The invention according to claim 5, wherein said voltage, frequency and spacing are related according to the formula:

wherein S is the distance between the groups of electrodes of said second set, V is the peak value of the voltage between said groups of electrodes, Ic is the mobility of the gas ions, and is the frequency.

7. The invention according to claim 6, including means for establishing a D. C. potential between said second set of electrodes and the main electrodes, of such value as to bring the second set of electrodes to a D. C. potential nearer to that of the discharge electrode than is provided by the normal D. C. gradient in the space between the main electrodes.

8.- The invention according to claim 7, wherein the second set of electrodes is located adjacent said collecting electrode of the main electrode.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,976,214 Brion et a1 Oct. 9, 1934 2,085,735 Brion et al July 6, 1937 2,086,063 Brion et a1 July 6, 1937 FOREIGN PATENTS Number Country Date 719,356 France Nov. 14, 1931 

